The present invention relates to an improved method of preparing bipyridinium salt by the coupling of 1-alkylpyridinium salt (expressed by the following formula I) with a base in the presence of an alkaline metal cyanide. The reaction mixture is added to a sufficient amount of an aqueous divalent transitional metal salt solution. After the co-precipitated divalent transitional metal cyanide and inorganic salt are filtered off, the filtrate is oxidized to afford 1,1'-dialkyl-4,4'-bipyridinium salt (expressed by the following formula II), free of cyanide ion and with low concentration of inorganic salts. ##STR1## (wherein R denotes an alkyl group having 1 to 10 carbon atoms; X denotes a chlorine atom, bromine atom, or a sulfate group. The compound of formula I, wherein R and X denote methyl group and chlorine atom, respectively, is the well-known pesticide paraquat.)
The preparation of 1,1'-dialkyl-4,4'-bipyridinium salt compounds by the alkaline metal cyanide--catalyzed coupling of 1-alkylpyridinium salts, followed by oxidation, is a published synthesis process. However, a environmentally sound treatment for the removal of the residual cyanides and inorganic salts has not been reported. U.S. Pat. Nos. 3,723,444; 3,787,426; 3,793,335; 3,868,381; 3,899,500; 3,905,986 disclose the preparation of 1,1'-dialkyl-4,4'-bipyridinium salt by reacting 1-alkylpyridinium salt with an soluble acid accepting agent (i.e. sodium hydroxide) in an organic solvent containing 5 to 50% of water at the presence of at least two equivalents of sodium cyanide. Such processes are uneconomical and inefficient on a commercial scale. For instance, the patents quoted above mention the of soluble alkaline metal salts of 4,4'-diamino-distyrene-2,2'-disulfonic acid (amsonic acid), with which the 1,1'-dialkyl-4,4'-bipyridinium cation is precipitated as an acidic salt and separated from cyanides by filtration. The salts are further treated with ion exchanged process to remove residual cyanic ions in order to afford a qualified product. Yet this is a very sophisticated and costly treatment process.
One other example discloses the addition of ferrous sulfate hydrate to the aqueous solution of 1,1'-dialkyl-4,4'-bipyridinium cation to precipitate cyanides, which are then removed by filtration. However, this process results in low yield of the final product. Another example discloses the extraction of the coupled precursor of 1,1'-dialkyl-4,4'-bipyridinium salt with an organic solvent such as toluene. The cyanic ion containing aqueous layer is phase separated, and the organic layer is then oxidized to give the 1,1'-dialkyl-4,4'-bipyridinium salt. Yet this process has the disadvantages of low recovery rate and cyanic ion-containing waste water.
In the present invention, the applicant has proposed a method of preparing 1,1'-dialkyl-4,4'-bipyridinium salt (formula II) by the coupling of 1-alkylpyridinium salt (formula I) in the presence of a proper amount of a cyanide and a base to afford the precursor of bipyridinium salt. The precursor is further oxidized to give the title compound in high yield. The applicant has further proposed a method of isolating the precursor by salting out the cyanide and inorganic salts in an aqueous solution of divalent transitional metal salt.